1. Field of the Invention
The present invention relates to a spread illuminating apparatus used as an illuminating means for signboards, various kinds of reflection-type display devices and the like, and more particularly for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display as a display for many electric products such as cellular phones and personal computers has been increasingly demanded due to its low profile, small occupied volume, and light weight. Since the liquid crystal display uses a liquid crystal, which does not emit light by itself, an illuminating apparatus is required when used in dark places where sufficient sunlight or room illumination is unavailable. Thus, the illuminating apparatus to illuminate the liquid crystal is desired to be compact and small in power consumption, and at the same time, high image quality is desired on the observation surface. Accordingly, there has been a growing tendency to employ a thin plate-like spread illuminating apparatus of side light type (light conductive plate type) as an illuminating means for the liquid crystal display.
FIG. 12 shows an example of a conventional side light type spread illuminating apparatus. This spread illuminating apparatus has been devised to realize a uniform spread light emission (Japanese Patent Laid-open No. 2000-11723), and the detail thereof is described below.
A spread illuminating apparatus 1xe2x80x2 is disposed so as to cover an upper face of a reflection-type liquid crystal element L, and is generally composed of a plate-like transparent substrate 2 shaped rectangular in section and made of a light-transmissible material, a light source 4 disposed close to one end face 3 of the transparent substrate 2, and a light reflection member (reflector) 12 to cover the light source 4 and a portion of the transparent substrate 2 near the one end face 3.
The light source 4 is composed of a bar-like light conductive member 7, and spot-like light sources 9 and 9xe2x80x2 such as light-emitting diodes disposed on both ends 8 of the light conductive member 7. An optical path conversion means 11 is formed on one face 14 of the light conductive member 7 opposite to a face 13 facing the end face 3 of the transparent substrate 2, and is composed of grooves 15 as light scattering portions shaped, for example, triangular in section, and flat portions 16 adjacent to the grooves 15. The interval of the grooves 15 is set to gradually decrease with the increase in distance from the spot-like light sources 9 and 9xe2x80x2. Thus, the optical path conversion means 11 is formed in consideration of a balance between the distance from respective spot-like light sources 9 and 9xe2x80x2 and the interval of the grooves 15, whereby light is uniformly emitted through the face 13 of the light conductive member 7 despite the spot-like light sources 9 and 9xe2x80x2 being disposed on the ends 8 of the light conductive member 7. The optical path conversion means 11 is not limited to the above configuration but may comprise grooves without flat portions or comprise light scattering portions with a finely roughened surface and plain portions with a smooth surface.
A light reflection pattern 17 is formed on an upper face 6 of the transparent substrate 2 in parallel to the end face 3. The light reflection pattern 17 is composed of a large number of grooves 18 substantially triangular in section and flat portions 19 adjacent to the grooves 18, and the interval between adjacent grooves 18 is varied from position to position so as to make the illumination uniformly spread everywhere on the transparent substrate 2 irrespective of the distance from the light source 4. Specifically, the ratio of the width (occupied area) of the grooves 18 to the width (occupied area) of the flat portions 19 is set to gradually increase in proportion to the increase in distance from the end face 3 of the transparent substrate 2.
Further, a light reflection member 12 is disposed so as to cover the longitudinal faces of the light conductive member 7 except the face 13 and the end of the transparent substrate 2 close to the light conductive member 7, whereby light emitted from the spot-like light sources 9 and 9xe2x80x2 into the light conductive member 7 is efficiently guided into the transparent substrate 2. The light reflection member 12 is made of a hard resin product substantially U-shaped in section, and having a metal (such as silver) vapor deposited film, a white film, etc. stuck on its inner surface, or made of a U-shape bent metal sheet such as an aluminum sheet and a stainless steel sheet, and disposed so as to cover the light conductive member 7.
However, the above spread illuminating apparatus has a disadvantage that dark and bright fringes orthogonal to the end face 3 appear on the screen. The dark and bright fringes are attributable to the optical path conversion means 11 formed on the light conductive member 7, which means that while light reflected at the grooves 15 of the optical path conversion means 11 is mostly emitted through the face 13 to be guided into the transparent substrate 2, light reflected at the flat portions 16 mostly undergoes total reflection and is not emitted through the face 13, and thus, the light is emitted through the face 13 with uneven brightness according to the pattern of grooves 15 and the flat portions 16. Since the brightness of light guided into the transparent substrate 2 is uneven, dark and bright fringes orthogonal to the end face 3 are generated on the screen. This problem can be solved if the pattern of the grooves 15 and the flat portions 16 is finely structured but it is difficult to ensure a machining accuracy required to realize such a fine structure.
So, it was proposed that a light diffusion plate 20 be disposed between the light conductive member 7 and the transparent substrate 2 as illustrated in FIG. 13 so as to uniform the brightness of light incident on the transparent substrate 2. The light diffusion plate 20 is structured such that a light diffusion portion containing a light diffusion substance is formed on a thin plate, and diffuses light emitted through the face 13 of the light conductive member 7 to substantially uniform the brightness of light incident on the end face 3 of the transparent substrate 2 (Japanese Patent Laid-open No. 2000-231814).
A spread illuminating apparatus 1xe2x80x3 shown in FIG. 13 is effective in uniforming the brightness of light on an observation surface but has a disadvantage that the transmittance of the light diffusion plate 20 is low, which degrades the ratio of utilization of light from the light source 4 resulting in an increased power consumption to obtain a required brightness on the screen.
However, without the light diffusion plate, the spread illuminating apparatus still suffers the problem that the dark and bright fringes cannot be fully eliminated on the screen even when the optical path conversion means is structured finely with highest machining accuracy.
The present invention has been made in view of the above, and an object of the present invention is therefore to provide a spread illuminating apparatus which can ensure uniform and high brightness entirely over the screen without increasing power consumption.
In order to solve the above problems, according to a first aspect of the present invention, there is provided a Spread illuminating apparatus which comprises a transparent substrate made of a light-transmissible material and a light source composed of a bar-like light conductive member and at least one spot-like light source disposed on an end of the light conductive member, disposed along and close to an end face of the transparent substrate, and having an optical path conversion means which comprises a large number of light scattering portions, such as grooves, with or without flat portions therebetween, and is provided on at least one face of the light conductive member, wherein a light reflection-diffusion layer is formed on the at least one face of the bar-like light conductive member.
In accordance with the present invention, the reflectance at the grooves is improved by forming the light reflection-diffusion layer on the face, whereby while light emitted from the spot-like light source is reflected toward the transparent substrate in an increased amount at positions closer to the spot-like light source, the intensity of light reflected toward the transparent substrate decreases substantially linearly as the increase in distance from the spot-like light source irrespective of the distance from the spot-like light source, and sudden and sharp change of brightness can be eliminated.
In order to solve the above problems, according to a second aspect of the present invention, in the spread illuminating apparatus of the first aspect, the light reflection-diffusion layer is a white coating, whereby the light reflectance can be improved, at the same time the intensity of light incident on the transparent substrate is adapted to degrade in a substantially linear manner with the increase in distance from the spot-like light source, and thus sharp change of brightness on the screen can be eliminated. Further, the uniformity in color on the observation surface can be improved.
In order to solve the above problems, according to a third aspect of the present invention, in the spread illuminating apparatus of the first aspect, the light reflection-diffusion layer is a light diffusible mat coating, whereby the light reflectance can be improved, at the same time the intensity of light incident on the transparent substrate is adapted to degrade in a substantially linear manner with the increase in distance from the spot-like light source, and thus sharp change of brightness on the screen can be eliminated.
In order to solve the above problems, according to a fourth aspect of the present invention, in the spread illuminating apparatus of the third aspect, the light diffusible mat coating is of a frosted mat. Further, according to a fifth aspect of the present invention, in the spread illuminating apparatus of the third aspect, the light diffusible mat coating is of silica.
In order to solve the above problems, according to a sixth aspect of the present invention, in the spread illuminating apparatus of any one of the first to third aspects, the light reflection-diffusion layer is formed entirely on the face provided with the optical path conversion means, whereby the light reflectance can be improved.
In order to solve the above problems, according to a seventh aspect of the present invention, in the spread illuminating apparatus of any one of the first to third aspects, the light reflection-diffusion layer is formed partly at predetermined portions of the face provided with the optical path conversion means, whereby sharp change of brightness is eliminated. Here, the predetermined portions are portions corresponding to dark portions of the uneven brightness conventionally generated on the screen.
In order to solve the above problems, according to an eighth aspect of the present invention, in the spread illuminating apparatus of the fourth or fifth aspect, the light diffusible mat coating is formed entirely on the face provided with the optical path conversion means.
In order to solve the above problems, according to a ninth aspect of the present invention, in the spread illuminating apparatus of the fourth or fifth aspect, the light diffusible mat coating is formed partly at a predetermined portions of the face provided with the optical path conversion means.